The significantly increased efficiencies of superconductors compared to other types of electrical conductors have caused superconductors to be considered for an increasing number of applications. Superconductors are electrical conductors that have virtually no measurable resistance to electrical flow below a critical temperature and under specific magnetic and electrical conditions. Specifically, superconductivity terminates if the magnetic field in the material is equal to or greater than a threshold field strength (known as the critical field), if the material carries a current equal to or greater than a threshold amount (known as the critical current), or if the material has a current density equal to or greater than a threshold amount (known as the critical current density).
Superconducting devices typically have two or three terminals. Both types of devices have a supercurrent flowing between two terminals. Unlike the two-terminal superconducting devices, however, the supercurrent in a three-terminal superconducting device is controlled by the use of a third terminal (e.g., either by applying magnetic fields or electric fields or by injecting electrical current at the third terminal).
Superconductors that are electrically connected by weak link junctions are particularly useful for a number of two and three-terminal superconducting devices. A "weak link junction" refers to a volume of a superconductor over which superconductivity is degraded (e.g., the superconducting wave function and therefore the critical current are depressed) and includes Josephson-junction types of weak link junctions. Exemplary applications for superconducting devices having weak link junctions include digital-to-analog converters, transistor devices, picosecond processors, and superconducting quantum interferometric devices (that detect and/or measure extremely small currents, voltages, and/or magnetic fields).
The discrepancy that exists between the critical current densities of the weak link junction and the regions of the superconductor removed from the junction is the basis for the above-noted devices. For example, the low critical current densities of weak link junctions enables them to be switched from the superconducting to non-superconducting states (analogous to the "voltage-on" to "voltage-off" states used in semiconductor devices) by passing small electrical currents through them; thus they are attractive for high-speed, low power switching applications. The current-carrying capacities of weak link junctions are also sensitive to small magnetic fields and thus a key component in devices used to measure extremely weak magnetic fields. Another use for weak link junctions is for the generation of high-frequency microwave signals (e.g., 72 GHz oscillators), which operate by applying a DC voltage to an array of weak link junctions. If the superconducting properties of the weak-link junctions are sufficiently uniform, the junctions couple to each other to produce a high-frequency signal.
In one weak link junction configuration, the weak link junction exists at the boundary between two or more superconductor crystals of different orientations (with the boundary being known as a high angle grain boundary). The disorder in the crystal structure at the boundary diminishes the magnitude of the supercurrent by breaking apart the electron hole pairs which create supercurrent. The breaking apart of the electron hole pairs decreases the number of electron hole pairs which can be propagated across the junction (relative to the superconducting regions away from the junction); thus decreasing the magnitude of supercurrent which can be propagated across the junction relative to the regions away from the junction.
Several methods exist to form high angle grain boundaries at the boundary between two superconductor crystals. One method is to fabricate bicrystalline superconductors on bicrystalline substrates (known as a bicrystalline junction). With this technique, one crystallographic axis in the first crystal is parallel to one crystallographic axis in the second crystal, and the other two crystallographic axes of the first crystal are rotated at an arbitrary angle relative to the remaining two crystallographic axes of the second crystal. In this arrangement, the surface of the bicrystal is perpendicular to the common crystallographic direction, and the superconducting film is deposited on the substrate surface. The weak link occurs at the junction of the bicrystal because the high-angle grain boundary formed at the bicrystal junction propagates into the superconducting film. This type of weak link junction is often referred to as an "in-plane" grain boundary junction because the grain boundary is formed via a rotation of the crystallographic axes on either side of the junction, about the normal to the substrate plane. There is considerable expense associated with preparing bicrystalline substrates, and the electrical properties of weak link junctions prepared using this technique are not sufficiently reproducible for many applications. Another drawback to using bicrystalline substrates is the weak link junctions must be aligned along the single row where the substrates are joined, thus the technique is not amenable for use in integrated circuit applications.
In another method for forming a high angle grain boundary, a seed layer, such as MgO, is used to control the crystal structure of the superconductor. The seed layer is deposited on a portion of a monocrystalline substrate; then a buffer layer, such as SrTiO.sub.3, is subsequently deposited on the top of the substrate and the seed layer, and finally a superconductor, such as YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO), is deposited on top of the buffer layer. The seed layer causes an in-plane rotation of the crystallographic axes of the portion of the buffer layer overlying the seed layer but no in-plane rotation of the portion of the buffer layer deposited directly on the substrate. The superconductor crystals form on the buffer layer with an in-plane orientation similar to that of the underlying portion of the buffer layer. As a consequence, the crystallographic planes of the superconductor crystals on either side of the boundary of the seed layer intersect at a 45 degree angle in the plane of the substrate (known as a biepitaxial junction). Like the bicrystalline junction, the angle between the crystallographic planes occurs only in the plane of the substrate, but the magnitude of the angle is limited to 45 degrees. The angle is limited to 45 degrees because this angle is the rotation which single-crystal seed layers adopt when deposited on the types of substrates (e.g., lanthanum aluminate, strontium titanate, yttria-stabilized zirconium oxide, and magnesium oxide) commonly used for the deposition of superconducting films.
The superconducting properties of bicrystalline and biepitaxial junctions have a high degree of variability and the properties of such junctions are therefore difficult to reproduce consistently in weak link junctions. The non-uniformity and wide variability of the superconducting properties of the junctions make the junctions unsuitable for many applications, particularly for applications requiring the use of multiple junctions. The adverse effects of the variability in the superconducting properties of such junctions are cumulative for multiple junctions. By way of example, in some applications hundreds or even thousands of weak link superconducting devices could be electrically interconnected.
In yet another method for forming a high angle grain boundary, a step is formed on the substrate using ion milling or chemical etching techniques, and the superconductor is deposited on the stepped surface. The high angle grain boundary between superconductor crystals results from differently oriented grains that nucleate at the surface step on the substrate (known as step-edge junction). The step size is typically 300 nm, and the superconducting film thickness is typically 100-300 nm. With step-edge junctions, the superconducting properties are critically dependent on the height of the step, the superconducting film thickness, and the quality of the superconducting film. Because it is difficult to control the magnitude of the step height, and because it is difficult to avoid damaging the crystalline quality of the substrate during the step fabrication process (which in turn degrades the quality of the superconducting film deposited on top of it), the step size and superconducting film properties typically fluctuate not only between steps on a given substrate but also among steps on a number of substrates. Because the superconducting properties of the step-edge junction are dependent upon the size of the step and superconducting film quality, the fluctuations cause the superconducting properties of step-edge junctions to be nonuniform and the properties of a given step-edge junction to be difficult to reproduce consistently. Accordingly, step-edge junctions suffer from the same problems as the bicrystalline and biepitaxial junctions.
There is a need for a weak link junction configuration that can be produced in large quantities with substantially uniform superconducting properties across the weak link junctions. There is a related need for a method to produce superconducting devices having weak link junctions formed by high angle grain boundaries that can be produced in large quantities with substantially equal angles at the grain boundaries.
There is also a need for a method to produce superconducting devices having weak link junctions formed by high angle grain boundaries that can produce a variety of different angles at the boundary. There is a related need for a method to produce such superconducting devices having a relatively low magnitude angle at the grain boundaries to reduce the amount of disordering at the boundaries to relatively low levels. There is a further need for a method to optimize the magnitude of the angle at the grain boundaries to produce the desired superconducting properties across the boundary while substantially minimizing the amount of disordering at the boundaries.